Although considerable research has examined general intellectual functioning and adaptive behavior in fragile X syndrome (FXS), only a few studies have examined specific domains of neurocognitive functioning, especially in males. Existing research has been limited by cross-sectional designs, lack of comparison groups, a wide age range in the samples, and inconsistencies in assessment methodologies. The proposed project would add substantially to our knowledge about neurocognitive function in FXS by conducting more detailed, systematic, and expansive assessments of attention, memory, and executive function. The project builds on an existing sample of children with FXS participating in a prospective, longitudinal study who are about to enter puberty. We will describe in detail the nature of cognitive function and development over time during early adolescence, and relate function and development to other clinical, genetic, neuropsychological, biological, and sociodemographic variables in order to describe more precisely the FXS phenotype and understand the variability within this population. The study will incorporate a sample of 68 children with FXS between the ages of 8 and 13 years. A comparison group of 68 mental-age-matched typically developing children will provide insights into relative cognitive strengths and weaknesses in FXS in relation to their own mental age. Each child will be assessed annually using a comprehensive battery of attention, memory, and executive function measures. Additional measures will be collected of FMRP, pubertal status, socioeconomics status, temperament, autistic behavior, language development, visual processing skills, and behavior problems. These additional assessments will test the relationship between protein expression and neurocognitive function, identify changes in patterns of development as a function of puberty, provide some insights into the role of environment in affecting outcomes, and increase the specificity of the FXS phenotype by providing a comprehensive description of a range of features likely to be associated with FXS. A range of analytic procedures, including hierarchical linear modeling, will be used to answer four specific aims related to determining neurocognitive profiles, comparing profiles with the comparison group, examining change over time, and testing the relationship between neurocognitive function and other variables of interest.